Interaction of radiation with a molecular resonance has found a variety of uses in emerging industrial applications, especially when (but not limited to) using a laser as the radiation source. Such applications require that the laser be tunable in the region of selected molecular resonances. Moreover, the laser must operate stably and with a spectral spread less than the line-width of the selected resonance. Some applications demand high-intensity repetitively pulsed output at sizable average powers; others require low-power lasers capable of CW operation. The applications include laser chemistry, material purification, atmospheric remote sensing, trace gas detection, chemical process control, and others.
There are several types of tunable lasers with broad amplification bandwidth in the spectral regions of interest. Two problems are generally encountered:
(1) the free running lasers tend to operate at a multi-mode spectral output spread over a broad interval, often considerably exceeding the line width of the molecular resonance. It is necessary to eliminate the multi-moding for the applications under consideration. PA1 (2) once single moding is achieved, the laser must be finely tuned to the frequency of a selected resonance, and maintained at that frequency over a long time period.
To review the situation we note that, e.g., a gain-switched (pulsed)CO.sub.2 laser at a multi-atmospheric gas pressure offers broad amplification band-width. Frequency tuning can be achieved with a resonator employing a tunable grating. Such a laser, however, has a multi-mode spectral output generally spread over a 2000 MHz interval. A variety of important uses require a laser frequency spread below 20 or 50 MHz. Similarly, a grating tunable near UV-pulsed excimer laser, as in KrF or ArF lasers, has a multi-mode output, sizably exceeding the frequency spread of a CO.sub.2 laser as described. A CW or pulsed dye laser with a simple grating resonator also operates at a spectral output spread over a broad region. The technology to extract the laser energy from such lasers at a finely controlled and pure frequency is a subject of considerable interest.
Elimination of laser multi-moding has been extensively scrutinized since the early days of lasers (the nineteen sixties). Several methods have been devised to achieve--with varying degrees of success--single mode operations at a tunable frequency. The recent efforts have been directed to high power repetitively pulsed lasers, operating at relatively high pulse energies.
Three principal methods have been devised to achieve single moding. One approach relies on operating the laser resonator at a wide mode spacing, so that when one of the modes is near the peak of the laser gain profile, the adjacent modes lie at sufficiently lower gains to allow the mode competition effect to inhibit multi-moding. In some cases a short laser resonator is used to obtain wide inter-order mode spacing. Other cases employ a compound resonator consisting of a short resonant structure coupled to a long laser resonator. In such cases considerable difficulties are encountered in seeking to achieve smooth tuning without a switch over to an adjacent mode. Such a mode-switching effect limits the frequency tuning to the vicinity of the maximum gain (or the peak of the grating response, if a resonator with a grating is used).
There have been other problems, e.g., a compound resonator requires difficult-to-achieve critical adjustments, unsuitable for use in a high power laser; a laser with a short resonator presents a small laser volume, unacceptable in cases where moderate or high laser energy is needed.
In another method, a short duration pulsed laser, operating in a transient regime, can be driven to oscillate at a single mode by means of transient regenerative amplification. A weak radiation field (obtained from an external tunable low-power laser), tuned to the frequency of a selected resonator mode of the pulsed (transient) laser, is introduced into the pulsed laser resonator. In that case, laser oscillation during transient build-up will occur at the selected single mode. Operation at a tunable frequency requires electronic control mechanisms to maintain coincidence of the selected resonator mode with the frequency of the driving (tunable) weak field. This approach has been successful and offers advantages where fine tuning is needed, e.g., high resolution probing of an absorption line-profile.
A third method, devised some time ago to obtain single-moding in a steady-state CW gas laser, utilizes the mode competition effect by employing an absorbing gas introduced in the laser resonator. The method relies on the nonlinearity of absorbing gas.
In most applications the tuning control is needed to bring the laser frequency to coincidence with an absorbing resonance belonging to molecules of interest (with which the laser radiation is to interact). Once the coincidence is achieved, the tuning control is kept at a fixed position, to maintain the coincidence over a long time duration. However, the reliance on a tuning control to achieve coincidence generally introduces frequency drifts caused by the tuning-mechanism drifts; this necessitates either frequent readjustments, or an automatic stabilization control.
In addition to other uses, as with broader band light sources, the invention described below makes it possible to extract laser energy at the frequency of an absorbing transition, without a need for broad tuning adjustment, as is necessary in a free running tunable laser. To illustrate, according to the invention there is provided an optical structure employed as a part of the laser resonator. The structure contains within it an absorption cell. To obtain laser operation at the frequency of an absorption resonance belonging to a gaseous medium, the user will introduce a quantity of the gas into the cell according to a simple prescription (dictating the pressure, and buffer gases where necessary). With that provision and a simple adjustment of the optical structure, the laser oscillation will occur within the line width of the selected absorption line. If the gas is removed from the absorption cell, the laser will cease oscillating; i.e., it will operate at the desired frequency, or there will be no laser oscillation. The invention is applicable regardless of the width of the absorbing resonance. The width can be the narrowest obtainable in the low pressure limit, corresponding to a Doppler broadened width. In important cases described below, the configuration employed will automatically eliminate laser multi-moding. Laser oscillation will occur on the single resonator mode nearest to the peak of the absorbing resonance.